The usual railroad car in use in the United States has an elongated body supported at each end by a truck containing wheels which ride on rails. Such cars in varying numbers are coupled together to make a train pulled by one or more locomotives. Braking railroad trains is almost always done by means of air brake apparatus, mounted on the individual cars, controlled by the engineer by means of an air line extending the length of the train and in communication with the braking apparatus on each car.
Many trains contain random consists of cars which are empty i.e. contain no lading, or are partially or fully loaded with lading. When the train engineer brakes such a train it is undesirable to have equal braking pressure applied to all the cars because thereby some may be over-braked and others under-braked. When over-braked, such as can occur when a car is empty or lightly loaded, the braked wheels may lock, stop rolling and slide along the track thereby causing flat areas to form on the wheels. Under-braking of wheels is also undesirable because this reduces the overall braking action applied to the train and makes some cars brake more than desirable with excessive wear on the brake shoes.
Because of the desirability of applying a predetermined braking force to wheels of loaded cars, and decreased braking force to empty cars, it is common to mount on the car body a single control valve system which regulates the pressure of air fed to the braking apparatus on the trucks at both ends of the car so that when the car is loaded, the air pressure is increased and causes a corresponding increase in braking force on the wheels. When the car is unloaded or empty, a reduction in air pressure occurs and braking force is reduced to a predetermined value.
While a braking control system as described has been found useful on conventional railroad cars of heavy construction it has not given precision braking control because the valve action requires a displacement of about 1.5 inches or so between the car body and the truck frame. This distance could not be significantly reduced due to car rolling which would activate the valve prematurely.
Another disadvantage of the described braking control system is that it incorrectly assumed the load on each of the two car trucks would always be equal whether the car was empty or partially or fully loaded. In some instances the car end where the control valve is located would be lighter or heavier than the other end, leading to incorrect braking at the end away from the valve.
In recent years major efforts have been made to lighten or reduce the weight of railroad cars. The braking of such cars, particularly when empty, requires less force than heavier cars so that braking apparatus and control systems which were useful for heavier cars would more likely over-brake the lighter cars and cause the wheels to lock and slide on the rails.
As part of the effort to reduce railroad car weight, the car builders and railroads have turned to some extent to building and using articulated cars, such as shown in U.S. Pat. Nos. 4,524,699; 4,233,909; 3,646,604 and 3,399,631. An articulated car has ends which are self-supported and hitchable to conventional non-articulated cars. An articulated car contains at least three units or platforms or elongated bodies with the adjacent ends of units connected together by an articulated connector supported by a single truck. In such a car there is always one more unit than articulated connectors.
Articulated cars at present are used primarily to transport containers and over-the-highway semi-trailers. In general, each unit or elongated body between hitches or articulated connectors is made long enough to carry the longest trailer or container in use although containers are very often double-decked.
In the case of container cars, a unit of such a car in a train may be (a) completely empty and not even carry one or two empty containers (b) carry one full container, with or without an empty container or (c) carry two full containers. Accordingly, the load carried by an articulated connector, joining two adjacent units of the car together, to the truck can vary from a minimum load when each unit is totally empty to a maximum load when each adjacent unit carries two containers with maximum allowable lading. The load on each truck in the articulated car, with the number of trucks being three or more, can vary within those extremes. Applying uniform braking force to the wheels of all the trucks as now done leads to simultaneous over-braking and under-braking of some truck wheels with undesirable results. A need thus exists for apparatus which will apply braking action correlated according to the load carried by a truck at the time of braking.